Production of crystalline materials by using high intensity ultrasound

ABSTRACT

A crystalline material sufficiently pure for use in pharmaceuticals may be made by forming a saturated solution of the material changing the temperature of the solution so it becomes supersaturated, and briefly subjecting the solution to irradiation by high intensity ultrasound, before allowing the solution to cool gradually without further irradiation. The ultrasound may be applied using a vessel with an array of ultrasonic transducers attached to a wall, so each transducer radiates no more than 3 W/cm 2  yet the power dissipation within the vessel is between 25 and 150 W/litre. This method can reduce the metastable zone width to less than 10 K. There is no erosion of the wall and consequently no formation of small particles of metal. It is applicable for example to aspartame, and to amino acids.

This invention relates to a method for crystallisation of ingredientsthat may be suitable for use in pharmaceuticals.

The use of high intensity ultrasound to trigger nucleation in asupersaturated solution, so that crystallisation occurs, is known, andan apparatus for this purpose is for example described in GB 2 276 567A. The benefits of triggering nucleation in this fashion are ofparticular relevance when very pure crystalline products are to beformed, as the purity of the solution and the cleanliness of the vesselsurfaces means that crystallisation nuclei are not otherwise present.Certain compounds would be desirable for use in pharmaceuticals, buthave been found particularly difficult to crystallise; this relates inparticular to disaccharides such as D-glucose or D-xylose. Similarproblems arise with other organic compounds such as aspartic acid, andthe compound α-L-aspartyl-L-phenylalanine methyl ester (aspartame). Ithas often been found necessary to add crystal modifiers to a saturatedsolution of such compounds to encourage the formation of crystals, as asaturated solution may have to be cooled considerably below thesaturation temperature before crystallisation occurs; with some organicmaterials this under-cooling may be as much as 100 K. That is to say, asupersaturated solution may remain in a metastable state for a prolongedperiod, which may be many months. The use of an immersed ultrasonicprobe or horn to subject a saturated solution to ultrasound is commonlyused, but it has been found that some cavitation occurs at the surfaceof the horn, this causing erosion of the horn and consequentialgeneration of very small metal particles (say about 0.1 mm in diameter);consequently this process would not be acceptable for generatingcrystalline material for use as a pharmaceutical ingredient.

Accordingly the present invention provides a method for production ofcrystalline material, the method comprising forming a saturated solutionof the material, changing the temperature of the solution so it becomessupersaturated, and subjecting the solution to irradiation by highintensity ultrasound, the ultrasound being applied only while thesolution is supersaturated, and being applied only until crystals areformed, and then allowing the crystals in the solution to grow withoutirradiation.

Preferably the ultrasound is applied for a time no more than 10 seconds,for example 2 seconds or 3 seconds. The ultrasound most preferably isapplied for a brief interval of say less than 5 seconds, and then thesolution inspected to see if any crystals have been formed; if nocrystals have been formed than the ultrasound may again be applied for abrief interval, and the solution again inspected. This may be repeateduntil crystals appear, after which ultrasound is no longer applied.Further gradual cooling of the solution, subsequent to the applicationof ultrasound, will lead to growth of the crystals formed during theultrasonic insonation. Hence this method enables large crystals to begrown.

The ultrasound may be applied to the supersaturated solution in a vesselusing a multiplicity of ultrasonic transducers attached to a wall of thevessel in an array extending both circumferentially and longitudinally,each transducer being connected to a signal generator so that thetransducer radiates no more than 3 W/cm², the transducers beingsufficiently close together and the number of transducers beingsufficiently high that the power dissipation within the vessel isbetween 25 and 150 W/litre. The values of power given here are those ofthe electrical power delivered to the transducers, as this is relativelyeasy to determine. Such an irradiation vessel is described in WO00/35579. Surprisingly it has been found that with such a vessel thereis no cavitation at the surface of the wall, so that there is no erosionof the wall and consequently no formation of small particles of metal.The crystalline material made by this method can be very pure, asadditives are not required and the crystallisation procedure does notintroduce contaminants, so that it would be suitable both for food useand for pharmaceutical use.

It is desirable to ensure no focusing of the ultrasound occurs, and thismay be achieved by energising groups of adjacent transducers insuccession. Where the vessel is cylindrical it is particularlypreferable to avoid energising diametrically opposite transducers at thesame time. The non-focusing can also be achieved by energising adjacenttransducers, or adjacent groups of transducers, at differentfrequencies; and in particular to vary the frequency at which eachtransducer or group of transducers is energized over a limited range,for example between 19.5 kHz and 20.5 kHz.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawingwhich shows a cross-sectional view of a batch crystallisationirradiator.

Referring to the drawing, a batch crystallisation irradiator 10 includesa stainless-steel vessel 12 of internal diameter 0.31 m and of wallthickness 2 mm. To the outside of the wall are attached sixty transducermodules 14 closely packed in a square array. Each transducer module 14comprises a 50 W piezoelectric transducer 16 which resonates at 20 kHz,attached to a conically flared titanium coupling block 18 by which it isconnected to the wall, the wider end of each block being of diameter 63mm. The transducer modules define five circumferential rings each oftwelve modules 14, the centres of the coupling blocks 18 being on asquare pitch of 82 mm. The irradiator 10 also incorporates three signalgenerators 20 (only one is shown) each of which drives the transducers16 in a pair of adjacent longitudinal rows and another such pair of rowsone third of the circumference apart from the first pair.

In use of the irradiator 10 the vessel 12 is filled with a solution andthe temperature of the vessel is gradually lowered (assuming thesolubility decreases as the temperature decreases) using a coolingjacket 22, and the contents of the vessel 12 are stirred. Consequentlythe solution will become saturated and then supersaturated. When thetemperature is about 10 K below that at which saturation occurs, thetransducers are energized briefly, each generator 20 being energized for0.8 second successively. Each transducer irradiates 50 W over a circleof diameter 63 mm, that is an intensity of 1.6 W/cm². The ultrasonicenergy is dissipated over the cylindrical volume of the vessel 12, whichis about 31 litres, so if all the transducers 16 were energisedsimultaneously the power density would be about 100 W/litre. To avoidfocusing, only one signal generator 20 is energized at any one time, sothe energy deposition is about 33 W/litre. After 0.8 second, a differentgenerator 20 is energized, and so on. After 2.4 seconds each transducerhas been energized, and application of ultrasound is terminated. Thecontents of the vessel 12 are then inspected, to see if any crystalshave formed. If there are no crystals this activation procedure isrepeated. Once crystals are observed, application of ultrasound isterminated, and the temperature of the vessel 12 is gradually lowered.

In a modification the signal generators 20 may generate signals at afrequency that varies between 19.5 and 20.5 kHz, the signals fromdifferent signal generators 20 varying independently of each other.

With this irradiator 10 the power intensity is such that cavitation doesnot occur at the surface of the wall, so erosion of the vessel 12 doesnot occur. Nevertheless the power density is sufficient to ensurenucleation in a saturated solution.

An experiment to investigate the effect of ultrasound on crystallisationhas been performed, as follows. An aqueous solution of D-xylosecontaining 25 g D-xylose per 10 ml water was prepared, which would besaturated at 50° C. This was than cooled at a rate of 0.2 K/min to 20°C., and the resulting solid products were separated and isolated. As acontrol, in one case the transducers 14 were not energized; in this casecrystals did not appear until the temperature had dropped to 36° C. Ifthe transducers 14 were energized for a period of 2 minutes, starting at46° C., then crystals appeared at 43° C. If the transducers 14 wereenergized continuously, starting at 50° C., then the resulting crystalswere very small, and information on sizes was not obtained. Table 1gives the temperature T at which solid first appeared and also shows theeffect on crystal size distribution by indicating the crystal size (inμm) for different cumulative percentiles (by mass):

TABLE 1 Conditions T/° C. 10% 50% 90% No ultrasound 36 27  67 149 2 minultrasound 43 43 106 211 Ultrasound 46 — — —Since the solutions were saturated at 50° C., ideally crystallisationshould commence as soon as the temperature drops below 50° C. The shortapplication of ultrasound markedly reduces the metastable zone width toonly about 7 K (as compared to about 14 K in the absence of ultrasound).It also gives a significant increase in the crystal sizes that areformed. Continuous application of ultrasound reduces the metastable zonewidth even more, to about 4 K.

It will be appreciated that the conditions that applied in thisparticular experiment do not exactly correspond to the method of thepresent invention, but that the results indicate that it would beappropriate to cool the solution to about 43° C. before subjecting it tobrief irradiation.

In performing the present invention, the temperature to which thesolution is to be cooled before the brief application of ultrasound willdiffer for different solutions, depending on the material, the solventand the concentration, and must therefore be found by experiment. It maybe ascertained by experiments similar to those described above. Thesolution is first subjected to continuous ultrasound as it is cooled,and the temperature at which crystals form (T, which in the exampleabove was 46° C.) is observed. Further tests are then carried out,cooling the solution to different temperatures within a few degreesabove or below T to find the highest temperature at which crystals formon application of a brief pulse of ultrasound. Typically this is within5 K of the temperature T observed with continuous ultrasound.

Aspartame is the α-dipeptide ester L-aspartyl-L-phenylalanine methylester and is an important synthetic low-calorie sweetening agent. It isabout 200 times sweeter than sugar and does not leave a bitteraftertaste, and so is used in a wide range of products. It is, however,difficult to crystallise without use of crystal modifiers, particularlyfrom aqueous solution. Surprisingly, it has been found possible toproduce satisfactory crystals of aspartame directly from an aqueoussolution using the present method. A saturated solution of aspartame inwarm pure water is prepared, and introduced into the vessel 12. Thetemperature of the solution is gradually cooled to about 10 K below thetemperature at which it would be saturated, and is subjected toultrasonic irradiation as described above for a short time, for example2.4 s. The solution is then inspected, and if crystals have formed as aresult of the ultrasonic irradiation, then the temperature of the vesselis gradually cooled over a period of a few hours down to roomtemperature.

This process has been found to produce aspartame crystals between 100and 250 μm in size, which are easy to separate from the remaining liquidfor example by filtration. By avoiding the need for additives the purityof the product is ensured.

The inspection to check if any crystals have formed as a result of theultrasonic irradiation may be an inspection by eye, while shining alight into the solution, as the small crystals sparkle.

It will be appreciated that the method is applicable using differentapparatus, and may be applied on a continuous rather than a batch basis.For example a saturated solution may be caused to flow along a duct inwhich its temperature gradually decreases, the duct incorporating aflow-through ultrasonic irradiation module at a position at which thesolution has reached the appropriate temperature, so that the solutionis briefly irradiated as it flows through the module. In this case thetransducers of the ultrasonic irradiation module might be activatedcontinuously or in a pulsed mode.

The method is applicable to many different chemical compounds. Forexample it may be used for proteins and amino acids, and forantibiotics. By way of example, the following measurements have beenmade with the three amino acids L-leucine, L-phenylalanine, andL-histadine.

Saturated solutions in water were prepared at 75° C., the concentrationsbeing 3.3, 6.2 and 11.3 g/100 g water respectively (after 24 hours incontact with solid material). Four samples were taken of each solution,and were then cooled at a steady rate of 0.2° C./min. In half the casesthe samples were subjected to a 10 s burst of ultrasound every 5 minutesuntil crystals were observed. No ultrasound was applied to the others.The temperature, T, at which crystals first appeared is shown in Table2, Tu being the cases with ultrasound, and Tx those without ultrasound.

TABLE 2 Tx/° C. Tx/° C. Tu/° C. Tu/° C. L-leucine 52.9 52.2 66.0 64.5L-phenylalinine 65.2 66.4 69.1 71.0 L-histadine 65.5 67.0 69.0 70.1

It will be appreciated that in each case the application of ultrasoundreduces the metastable zone width, so the crystals appear at a highertemperature. The effect is most dramatic in the case of leucine, wherethe metastable zone is decreased from about 22.5 K to about 9.8 K. Inaddition, the ultrasound has an effect on the crystal size distribution,so that the crystals are larger. For example Table 3 shows measurementsof the resulting crystal size distributions, as measured with a MalvernMastersizer 2000, for histadine and phenylalanine, showing the particlesizes (in μm) for different cumulative percentiles (by mass).

TABLE 3 10% 50% 90% Histadine no ultrasound 15.8 59.9 166.3 Histadineultrasound 20.6 89.6 370.8 Phenylalanine no ultrasound 133.9 352.4 655.9Phenylalanine ultrasound 159.9 420.9 776.1

As another application, a saturated solution may be insonated so as togenerate crystals, and then be added to a larger volume of solution sothat the crystals act as seed crystals for the entire volume. Forexample there might be 4000 litres of a saturated solution in acrystallisation tank, which is gradually cooled or to which anti-solventis added. When it is sufficiently supersaturated, a small quantity (eg40 l) is transferred into an irradiation chamber (eg sucked up through apipe) at the same temperature as the tank; there it is subjected toultrasound so that crystals are formed; it is then transferred back intothe tank. If no crystals are formed, this operation may be repeated.

1. A method for production of crystalline material, the methodcomprising forming a saturated solution of the material, graduallychanging the temperature of the solution so said solution becomessupersaturated, and subjecting the solution to irradiation byhigh-intensity ultrasound, wherein the method involves: a) finding thetemperature which provides the least degree of supersaturation at whichcrystals form from the solution on brief application of ultrasound, andthen b) subjecting the solution while it is supersaturated, at thistemperature, to ultrasound at a power intensity between 25 and 150W/litre for a brief interval no more than 10 s such that crystals areformed, and then c) allowing the crystals in the solution to growwithout irradiation by continuing to change the temperature of thesolution.
 2. A method as claimed in claim 1 wherein the solution issubjected to ultrasound for less than 10 s.
 3. A method as claimed inclaim 1 wherein the ultrasound is provided to the supersaturatedsolution in a vessel using a multiplicity of ultrasonic transducersattached to a wall of the vessel in an array extending bothcircumferentially and longitudinally, each transducer being connecLed toa signal generator so that the transducer radiates no more than 3 W/cm²,the transducers being sufficiently close together and the number oftransducers being sufficiently high that the power dissipation withinthe vessel is between 25 and 150 W/litre.
 4. A method as claimed inclaim 3 wherein ultrasound is applied in such a way that no focusing ofthe ultrasound occurs.
 5. A method as claimed in claim 4 whereinfocusing is prevented by energising groups of adjacent transducers insuccession.
 6. A method as claimed in claim 4 wherein focusing isprevented by energising adjacent transducers, or adjacent groups oftransducers, at different frequencies.
 7. A method as claimed in claim 1wherein the crystalline material is aspartame.
 8. A method as claimed inclaim 1 wherein the crystalline material is an amino acid.